FIG. 1 shows a common assembly of pipe hood and pipe unit at the air inlet and outlet openings in a ventilating fan. FIG. 2 is a schematic view of FIG. 1 showing the internal construction thereof. As shown in FIG. 1, the pipe hood 10 is made of a front cover 11 and a rear cover 12. A horizontal baffle 21 is provided inside the pipe unit 20 to form an upper air outlet pipe and a lower air inlet pipe 22. Corresponding to the baffle 21, the rear cover 12 is also provided with a horizontal partition plate 120 to divide the space enclosed by the front cover 11 and the rear cover 12 into an upper air outlet side 112 and a lower air inlet side 113 and form an internal air outlet opening 130 and an internal air inlet opening 140. An external air outlet opening 115 is formed on the upper portion of the front side of the front cover 11, and an external air inlet opening 114 is formed under the front side of the front cover. When the ventilating fan is in use, the air discharged from the ventilating fan enters the air outlet side 112 via the air outlet pipe 23 and the internal air outlet opening 130, and then is discharged to the outdoor via the external air outlet opening 115. Simultaneously, the fresh air introduced by the external air inlet opening 114 from the outdoor enters the air inlet side 113, the internal air inlet opening 140 and arrives at the inside of the ventilating fan via the air inlet pipe 22. Therefore, an air outlet passage and an air inlet passage are formed vertically on the upper side and the lower side in a vertical direction as shown in FIG. 2, respectively.
However, because the external air outlet opening 115 and the external air inlet opening 114 are disposed on the upper and lower sides of the front side of the pipe hood 10 respectively, when the air is blown from the front side or from the upper side, the air discharged from the external air outlet opening 115 may enter into the indoor room via the external air inlet opening 114 and cause a secondary contamination of the air to be fed into inside of the room.
Further, because the external air outlet opening 115 is positioned on the front side of the pipe hood 10 and the switching curtains at the external air inlet opening 114 and external air outlet opening 115 maintain a fixed structure in use, the rain, foreign object and the like are likely to enter into the pipe hood 10 via the external air inlet opening 114 and the external air outlet opening 115 and thus enter into the ventilating fan and entails greater potential safety trouble.
Further, because the inside of the pipe hood 10 needs to be cleaned regularly, the front cover 11 needs to be disassembled frequently. However, there is no satisfactory snapping structure between the front cover 11 and the rear cover 12.
Further, such a conventional design does not takes into account of the fact that the temperature of the discharged air is usually higher than the introduced air, and therefore, there is no heat exchange in the whole structure and thus the energy is wasted.